This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Final Fantasy XIV
Legacy / A Realm Reborn (1.0-2.55)
The Ultima Weapon's entrance. You think you've beaten Garuda, but her subjects' worship makes her unbeatable. Then Gaius appears, and taunts her. She responds by forcing the captive beastmen of the other primals to pray to Ifrit and Titan, and with a push from Garuda, they manifest right then and there. All three primals you have fought are in one place. Couldn't get any worse, right? But then Ultima Weapon drops in. Ifrit attacks it first... and is defeated in seconds. And is then absorbed into Ultima Weapon. It then makes short work of Titan, absorbing him too. Garuda is the only one left. Up until now, Garuda hasn't shown anything other than contempt for everything other than herself. But faced with Ultima Weapon, she expresses pure fear for the first time. But she's unable to stop the inevitable; once Ultima Weapon gets a hold of her, it crushes her head, absorbing the third primal. This thing has the power of three primals, and is in Garlean hands.
Tempering. Not only can a primal convert someone to their side in a burst of power — with no way to prevent, dodge, or block it, unless you have the Echo — there is no known way to reverse the process. Tempered victims are actually routinely killed off by the good guys as a result of this, because they view it as a Fate Worse Than Death and leaving them alive only serves to potentially strengthen the primal that has them under control. Worse, while tempering immediately twists loyalty to the primal, Color-Coded for Your Convenience isn't necessarily enforced. Tempered sylphs are generally identifiable by their purple coloring instead of green (although they still have the power to shapeshift so even that isn't a certain thing), but we see friendly Amalj'aa with red clothing that help new Black Mages, and tempered soldiers wearing the clothes of their country as they commit treason. The Beastmen tribe and Summoner quests only augment the horror here : *anyone* can be tempered or claim someone else to be tempered.
What's worse, is that it's known that staying in an area that's strongly aspected in one type of aether (such as water or fire) often causes harmful effects. For those who cross the point of no return, if they're lucky, they'll suffer a quick death with as little pain as possible. If they're unlucky, they'll begin suffering (painful) mutations caused by overaspecting in that aether, which will likely kill them, just more painfully. As the Tempering process involves the Primal giving a burst of power with the aether they are attuned to, this protects them from most of the lethal affects of overaspecting but still has its other drawbacks of causing disfigurement, mutations, and pain if the Primal so chooses. Of those tempered by Ifrit, it seems the most extreme we've seen thus far is some "Tempered" enemies near Za'harak appearing to be ashen and soot covered with eyes as black as coals. For Leviathan's "Drowned", as seen in Sastasha (Hard), this goes as far as him inflicting pain with it and causing those who have failed him to become more and more disfigured, replacing their heads to become more Seakin creature in appearance, such as that of a giant squid or jellyfish like, while the courtesans and serving ladies who were loyal to those pirates have been turned into lahmias (snake/fish bodied creatures) and pleading with or getting enraged at the players seeing their new appearances. Let's be glad that the other Primals we've seen thus far are not as interested in Tempering the spoken races for their own reasons.
The hedge sculptures in Haukke Manor's courtyard. Looming humanoid figures with holes through their heads — a reminder that the lady of the hous has been murdering beautiful young women and shredding their faces beyond recognition.
And then Hard mode makes it worse, when we're shown just how easy it is to turn a normal person into a demon with the Succubi turning Wood Wailers, who went to clean up the place after you were last there of any remaining threats, via their Demonize power. Now think back to some of the enemy names from the original Haukke Manor mode...yeah.
The release of Heavensward has added another layer of horror to this tale. Deep inside the Great Gubal Library is an old journal which reveals that its author, a Sharlayan scholar, was the one who convinced Lady Amandine to transform herself into a Succubus, and even helped oversee the ritual. He quickly realized he had made a terrible mistake once the ritual was complete and voidsent began overtaking the manor.
A great deal of The Lost City of Amdapor certainly qualifies. For anyone familiar with Nausicaš, the ruins in this dungeon bear a striking similarity to the forgotten kingdoms grown over by the toxic jungle, and you can plainly see the horrifying corruption spread far into the distance - what isn't obscured by clouds of choking spores, at any rate. Worse yet is the initial boss, a half-decayed, still-living goobue that routinely devours your fellow party members. If you don't kill it fast enough, the lifeless corpses of your comrades will be regurgitated onto the pock-marked stone flooring. And then you get to the interior of Amdapor, seeing the marvelous architecture that once made up the whole city, and discover Diabolos at the end...and then that music starts playing...
The previews for Tam-Tara Deepcroft (Hard) showed the final boss arena to be littered with pentagrams, runes, and the name "Avere" repeated over and over again in Eorzean script. Avere was the name of the tank of an NPC party who shows up during the initial main story Tam-Tara questline. He died because his fiancee Edda, a Conjurer, couldn't keep up on heals. She originally kept his severed head so that she could give him a burial...but all signs point to Edda trying to resurrect him using Blood Magic.
Later previews showed Edda with a giant, terrifying, demonic-looking head that has vaguely Ahriman-like features, except that its arms are its nerve endings.
Previews also showed Edda self-harming in order to power up said head. The animations for it are...unsettling.
Later, once you do the dungeon (and are in a party willing to read the 'Torn Folio' lore items), Edda specifically references the player character, giving an added touch of skin-crawling regret for earlier sympathies, and shades of My God, What Have I Done?.
The ending of the quest, "Corpse Groom": After you defeat the Avere-head, Edda accidentally falls from the platform to her death. You go outside to assure her old teammate that it's all over — and he looks across the chamber to see Edda standing there looking at him with the creepiest◊Slasher Smile. The way Edda appears wouldn't look out of place in Fatal Frame. He promptly flees, you turn around — and she's not there. Sure, Paiyo's exaggerated facial reaction's kinda funny, but still...
Another terrifying thing is the way she dies. Not the fact that she fell, but the fact that she's smilingas she falls...◊
And worse? Yoshida has stated that he isn't quite finished with Edda's story, which means we haven't seen the last of her.
She can occasionally appear for a few seconds in any of the three main towns. One of the places she can appear is in the Acorn Orchard in New Gridania. The Acorn Orchard is a playground filled with children.
Could also double as a Tear Jerker, however, as Edda might be looking at the life she and Avere could have had but will never have again.
Midgardsormr evokes this easily. For 15 years, everyone has presumed he's been dead and his charred body is a reminder of the first major defeat, and failure at invading Eorzea, for the Garlean Empire, at the terrible cost of transforming the once beautiful river, waterfalls, and forest-filled Mor Dhona into the landscape it is today. In reality, he's just been asleep for all these years, quietly observing and calling to his followers and children, none to happy for the actions Ishgard has taken against Dragonkind. His first act upon beginning to stir awake is to begin calling his children to gather the Dragon Horde together to prepare for a massive assault on Ishgard. His second act is to question the Warrior of Light's accomplishments while easily stripping them of Hydaelyn's protective light, draining the power of the 6 elemental crystals they've gathered and forcing them into a covenant with him. Oh sure, you've still got free will, but from here on out the Mother Crystal isn't going to come bail your rear out of another no-win situation like she did with protecting you from Ultima and neither is your new "friend".
Worse yet, because you are now somehow bound to the Guardian of Silvertear you are, by all accounts, a Heretic according to Ishgard culture. The only reason you aren't considered one yet is because the Scions and Midgardsormr won't, or haven't, revealed this fact to them so far. If they do, the fragile alliance with the Scions and the City-states with Ishgard WILL break and then you'll be dealing with yet another faction as your enemy.
On the other hand, given that as of Patch 2.55 Estinien, who is similarly bound to Nidhogg, is cooperating with Ishgard against the Dravanian horde, the chances of you being declared a Heretic have decreased.
Also, comparison between the English and Japanese versions of the dialog makes it clear that according to Midgardsormr, Hydaelyn has no issue with this and it was apparently part of a pact they had previously made. Meaning the Mother Crystal is willingly forsaking you in order to let Midgardsormr test you, simply because he asked her to.
Nabriales' fight. Up until now you've been protected by Hydaelyn. But, thanks to Midgardsormr, that's no longer the case. Additionally, unlike Lahabrea, he has no need to take things lightly on you nor does he taunt you into risking to have to kill a friend of your own to temporarily banish him. You get to find out just how much the Ascians at this point have been holding back their true power. And despite you finding out that the Scions have had the tool needed to call forth a massive amount of aether all this time in their possession, it still isn't enough to destroy a trapped Ascian, who can very much fight back even when sealed in a White Auracite gem. It takes the Heroic Sacrifice of Moenbryda to give you the extra aether needed to destroy Nabriales. And if Midgardsormr's words are anything to consider, hers might not be the last death needed to repeat this process to other Ascians.
Even Hildibrand, the side story meant to be lighthearted and hilarious, is not immune to this. In 2.5, we learn that Ul'dah's shady dealings even extend well into the past when the ancient nation of Belah'Dah split into Ul'Dah and Sil'Dah. Namely, Ul'Dah created "Trader's Spurn", aka zombiefication powder, which turns anyone struck by it into a zombie. And they used it during their war with Sil'Dah. Then, just to Kick the Dog a little more, they set up a secret organization known as the "Arbiters", whose role was to alter the historical records to make Ul'dah look good by blaming the creation and use of Trader's Spurn on Sil'Dah. The truth, when revealed, horrifies one of the Arbiters so about what's been hidden in the past she resigns her position and calls for reformations, if not the total removal, of the Arbiters.
To rub more salt in the wound, those ruins you see near the Golden Bazaar and by the edges of the Sagoli Desert? That's probably what was left over during the war and the zombies you see roaming around are likely the former residents of Sil'Dah, who are now wandering aimlessly, still seeking revenge on Ul'dah (which is probably why they attack the players who wander by).
Additionally, we learn from Gilgamesh just how frighteningly easy it is to summon and create a primal. It does not require any pre-established "godhood" or "saint" status with a flock of believers. It just takes Crystals, a strong desire, and prayer for any being to exist. And it takes as little as one person, with about a dozen and a half crates of crystals to do so. It may not be as strong as the traditional idea of what a Primal is, but a formidable foe nonetheless.
On the subject of Primals, 2.5 gives us a proper Odin Trial. And what do we learn from it? Odin is possibly not the True primal, rather his sword Zantetsuken is. And each time Odin is defeated, as Zantetsuken is immensely aetherically dense, no one gives pause to wonder why that doesn't disappear, figuring it's just an exotic metal weapon. Meanwhile, Zantetsuken just slowly gathers aether back to itself and forms a new wielder and body. Worse yet, it takes just any fool who wants to wield the sword itself to touch it to be instantly tempered by it.
Fighting Cerberus, a 3-headed monstrosity, in the World of Darkness is unsettling enough by itself, but once he breaks free from his chains he gets a lot more aggressive. The beast hurls up what looks like purple vomit and standing in it makes the monster instantly rush at you and tear you to shreds for a One-Hit Kill. If you managed to get shrunk by the Gastric Juice and go in the vomit, Cerberus swallows you alive and the game actually transitions into a new "area" where you get to wander around inside the boss' stomach and it pulsates to boot. Now you're slowly taking damage (and it builds up over time) from the stomach acids and blob creatures with one too many eyeballs spawn and attack you. The only way out is to damage the stomach walls enough until you're forced out through regurgitation. During your time in the stomach, the battle music gets quieter and muffled to add to the creepy factor and you're completely separated from the alliances that are still fighting the creature from the outside.
Also, if Cerberus dies with people inside, he won't regurgitate. Players can use Return or wait for the game to push them out, but the first time it's pretty jarring. Fortunately, this was fixed in a hotfix shortly after 2.5's release and the players are pushed out as soon as Cerberus dies.
Raubahn in 2.55's story. Up until now, Raubahn has been a fairly stoic character and can handle almost any situation pretty rationally and calmly, but 2.55 shows what happens if you push him to far. The Sultana is killed, and Raubahn is in complete grief and denial about her death. Meanwhile, Teledji Adeledji is taunting him about burying the sultana and how she must have felt gracious that someone "cut her strings." Raubahn has none of it and proceeds to cleave Teledji in two, all while showing a glimpse of his face that almost seems demonic while he does it. He targets Lolorito next, seemingly bent on going on a murderous rampage, but gets his arm cut off by Ilberd. And then, after Ilberd admits to killing the Sultana, he flies into yet another murderous rampage and lets out an inhuman roar. Even though he regains his senses and helps the Warrior of Light and the Scions escape, this drastic character shift in Raubahn is unsettling.
Nanamo's death scene is utterly chilling. One moment you're having a rather nice chat with the Sultana over her abdication of the throne and her plans for the future of Ul'Dah, one part of said plan being your support of Raubahn as her would-be idea for a republic roughs itself out. Then she takes a slow sip of her wine...and then it hits. Her likely last healthy heartbeat rings out as her eyes go wide, the poison taking effect as she silently chokes and reaches out to the Warrior of Light for help, before slumping over onto the floor, dead. It's absolutely jarring seeing one of the most kind hearted and unanimously good characters you've come to befriend and likely respect be murdered so gruesomely. The fact that it's the framing for a series of Wham Episodes doesn't help anything.
The fat chocobo minion pet is adorable and tumbly, then you get it and read its minion description in the journal. How did it get that fat as a hatchling? It cannibalized its siblings.
While you never get to witness it, Novv tells you his backstory when you gain the rank of friendly in the Sahagin beast tribe and it's quite chilling. Novv was the infamous Scarlet Sea-Devil during his heyday and when he came back home one day, he found his entire clutch completely slaughtered with bodies of his children piled on top of each other while only a few hidden eggs survived. Novv couldn't do anything but howl and cry until he had no energy left to mourn, and from there he decided to take his remaining unborn children and move elsewhere so he can raise them away from the violence while retiring from his pillaging and killing ways. You can only imagine the anguish Novv felt seeing his family taken from him with their bodies serving as a reminder for what he had done.
The Dark Knight quests. Despite having survived many ordeals (which may have included the Calamity itself, a war with the Garlean Empire, the beast tribes constantly summoning primals, and even the betrayal in Ul'Dah by the Monetarist and Crystal Braves, the player character/Warrior of Light has remained stoic through it all. Then, one day, they come across a fallen knight in black armor and find a Soul Stone. Nothing at all unusual to them as the WoL at this point; Heck, the skills and transfers of abilities to them will probably be useful, right? Well, this soul stone reacts much more differently than the others before and, after blacking out, the WoL finds the fallen knight is somehow alive and well, and knows them by name. You, in character, don't question this. Matter of fact, you almost get a sense of familiarity from this "Fray". At first, there's nothing too unseemly, but as you delve further and further into your new Dark Knight powers, people start noting you've changed a bit, and are acting differently now. Then the Wham Episode hits, and Fray reveals they are you, or rather, the persona of your psyche that has suffered pain, loss, anger, betrayal, fear...some of which are from people whose lives you've saved. You aren't the invincible, unflappable hero you think you are and you, the WoL, can fall into darkness just as easily as anyone else can.
And then there's the completely insane noblewoman that replaces Fray as the questline's Big Bad.
At the very end of the questline, Ystride, said noblewoman, gives you, Riella, and Sidurgu a chillingly demented Motive Rant/"The Reason You Suck" Speech. She madly claims that Riella's death is "the will of the Fury" before laughing maniacally. And finally, just to cement how truly far past the Moral Event Horizon she is, her last act before Sidurgu finishes her off is to give a deranged Slasher Smile.
The cutscene you get if you fail to defeat Bismarck before he wrecks the island you're fighting him on. You get a special cutscene where your character is in pain and unable to keep fighting, and they look towards the other direction to see Bismarck flying towards them, mouth wide open. As the screen fades to black, the last things you hear are ground breaking followed by a swallowing sound. This is, so far, the only cutscene in the game where it actually shows your character die.
Azys Lla, full stop. It's essentially the Allagan version of Big MT, and is where they conducted the research behind almost all of the horrific, mechanical or gene-spliced monstrosities you've faced to date. The fact that the various interfacing nodes are entirely cavalier about what went on there (and that one of the areas is a museum, meaning this place was a tourist attraction before Allag's fall) demonstrates just how far gone the Allagans were, culturally, by the end.
The nodes also demonstrate that the Allagan would randomly conscript citizens for dangerous tasks, with the worst example being a node that recruits test subjects for the city's "compliance systems". The node expresses disappointment that the robots failed to kill you.
Worst of all, this place is also the Can for The Warring Triad, and the aftermath of Thordan's actions are allowing them to break free. Eorzea might be this close to another Calamity, this time involving three Elder Primals
The Vault Conspiracy in 3.1. As it turns out, Ystride was far from the only dangerously unstable member of Ishgard's clergy. The absolutely insane lengths the Vault Priest and Ser Seninough go to in a desperate attempt to deny the truth of the Dragonsong War and to undermine Aymeric's reforms are downright chilling.
The demented Slasher Smile The leader of the conspiracy makes when he tries to throw an innocent girl to her death. All while madly claiming that her death will be your fault for your "heresy". Hilda mentions if you speak to her after completing 3.1, that despite whatever methods the Inquisition use on him, the Priest refuses to confess and tell who the other conspirators are. And that apparently he still has that deranged smile on his face to this day.